The present application relates to a data processing apparatus and method that perform detection of correlated positions of two data series, a receiving apparatus and method that perform synchronous detection by performing correlation operation of synchronous words composed of known series added to a received packet, a synchronous detection apparatus and method, and a computer program. In particular, the present application relates to a data processing apparatus and method that can perform synchronous detection with a small amount of operation even in a communication environment in which the frequency characteristics are greatly changed or which has a great non-linearity, a receiving apparatus and method, a synchronous detection apparatus and method, and a computer program.
Non-contact communication methods that may be applicable to RFID may be given as an electrostatic coupling type, an electromagnetic induction type, a radio wave communication type, and the like. Also, RFID systems may be classified into three types in accordance with transmission distances: a close coupled type (equal to or less than 0 to 2 mm), a proximity type (equal to or less than 0 to 10 cm), and a vicinity type (equal to or less than 0 to 70 cm), which have been prescribed by International Standards, such as ISO/IEC15693, ISO/IEC14443, ISO/IEC15693, and the like. Among them, as the proximity type IC card standards based on ISO/IEC14443, type A, type B, and FeliCa (registered trademark) may be given.
Further, NFC (Near Field Communication) developed by Sony Corporation and Philips Corporation is mainly an RFID standard which prescribes the specification of an NFC communication device (reader-writer) that is communicable with respective IC cards of the type A and FeliCa, and has become International Standard as ISO/IEC IS 18092 as of December 2003. The NFC communication method has been succeeded from “FeliCa” of Sony Corporation and “Mifare” of Philips Corporation, which have been widely spread as non-contact type IC cards, and uses a band of 13.56 MHz to make the proximity non-contact type bidirectional communication of about 10 cm possible by the electromagnetic induction method (NFC prescribes active type communication of a reader-writer in addition to the communication between a card and a reader-writer).
The primary use of the non-contact communication in the related art is billing or personal authentication, and the communication rate of about 106 Kbps to 424 Kbps is sufficient for the non-contact communication. For this, in order to perform the exchange of large capacity data with the same sense of access time in the related art in consideration of applying to diverse applications such as streaming transmission and the like, a highly accelerated communication rate is necessary. For example, in the FeliCa communication, 424 Kbps, 848 Kbps, 1.7 Mbps, 3.4 Mbps, and the like, which are multiples of 212 Kbps, have been provided, and 212 Kbps and 424 Kbps are mainly used at present. However, it will be considered hereinafter to further increase the communication rate to 848 Kbps, 1.7 Mbps, 3.4 Mbps, and the like.
However, in most communication systems and storage systems, a packet exchange method has been adopted. Its primary purpose is to avoid the occupation of a transmission path by handling the data in a small unit that is called a packet, to efficiently share the communication lines, and to be able to efficiently cope with a path to be replaced when an obstacle occurs in a portion of the communication path.
In packet communication, it is necessary to perform synchronous processing whenever a packet reaches a receiving side. In general, the known series is included in a header section of a packet, and a receiving side can acquire synchronous timing by performing a correlation process of the known series.
FIG. 6 illustrates a physical layer format of a packet prescribed in the NFC standard as described above. As illustrated, the packet is composed of three parts: ‘a preamble part’, a ‘synchronous (SYNC) part’, and a ‘data part’. The preamble part is composed of 6-byte length series of “0”, and the sync part is composed of a synchronous word which is composed of two-byte known series of “0xB24D”. Also, the data part includes a one-byte LEN that indicates the packet length, a (LEN-1)-byte length data body (i.e. payload), and a two-byte CRC (Cyclic Redundancy Check) code. These parts are all Manchester-encoded. In this case, the same packet format is used in a downlink and an uplink.
On the receiving side, the synchronous processing is very important. According to the format as shown in FIG. 6, the synchronous timing can be detected in the sync part, and it becomes first possible to decode information of the following LEN and payload. That is, if the synchronous processing is not accurately performed, it may not be possible to receive the whole packet.
FIG. 7 schematically illustrates a functional configuration of the synchronous processing. A hard decision unit 71 performs hard decision of a received sample every time n, and outputs a hard decision value of −1 or +1. A correlation operator 72 calculates mutual correlation between a synchronous word series for reference that is composed of N-word length and a hard decision result series composed of N words after the time n. Then, a comparator 73 outputs timing in which the mutual correlation value exceeds a threshold value R within a predetermined search window as detection position information of the synchronous word.
If it is assumed that the synchronous word length is N, synchronous words for reference are {ai}, aiε{−1, +1} (where i is an integer in the range of 0 to N−1), and received samples at time n are {yn}, the mutual correlation values calculated by the hard decision of the received samples are expressed as in Equation (1).
                                                        X              ^                        n                    ←                                    ∑                              i                =                0                                            N                -                1                                      ⁢                                                  ⁢                                          a                i                            ⁢                                                y                  ^                                                                      n                    -                                          (                                              N                        -                        1                                            )                                        +                    i                                    ⁢                                                                                                                            ⁢                                  ⁢                  Where          ,                                          ⁢                      {                          a              i                        }                    ,                                    a              i                        ∈                                          {                                                      -                    1                                    ,                                      +                    1                                                  }                            ⁢                              :                            ⁢              synchronous              ⁢                                                          ⁢              words              ⁢                                                          ⁢              for              ⁢                                                          ⁢              reference                                      ⁢                                  ⁢                              {                                          y                ^                            n                        }                    ,                                                    y                ^                            n                        ∈                                          {                                                      -                    1                                    ,                                      +                    1                                                  }                            ⁢                              :                            ⁢              hard              ⁢                                                          ⁢              decision              ⁢                                                          ⁢              values              ⁢                                                          ⁢              of              ⁢                                                          ⁢              received              ⁢                                                          ⁢              samples              ⁢                                                          ⁢                              y                n                            ⁢                                                          ⁢              at              ⁢                                                          ⁢              time              ⁢                                                          ⁢              n                                                          (        1        )            
Also, FIG. 8 shows in the form of a flowchart a processing order for performing synchronous word detection on the basis of the mutual correlation result between the synchronous reference word having a length of N and the received word calculated from the hard decision value series of the received sample having a length of N in the same manner.
First, the detection start time n of the synchronous word is designated as an initial value no (step S1). Then, a mutual correlation operation by the hard decision is performed using Equation (1) (step S2), and then it is determined whether the calculated mutual correlation value is equal to or larger than the predetermined threshold value R (step S3).
At this time, in a system which adopts a format that does not manage the positive or negative polarity such as NFC, it is necessary to compare the mutual correlation value with the threshold value as absolute values as in the diagram.
Here, if the mutual correlation value has not yet reached the threshold value R (“No” in step S3), it is determined that no synchronous word exists in the neighborhood of the received sample at time n. In this case, the synchronous word detection start time n is incremented, and then the processing returns to step S2 (step S4) to continue the mutual correlation operation.
On the other hand, if the mutual correlation value exceeds the threshold value R (“Yes” in step S3), it is determined that the synchronous word has come. In this case, the synchronous word detection time n is output (step S5), and then the corresponding processing routine is ended.
In the synchronous processing as shown in FIGS. 7 and 8, the error rate of the hard decision of −1 or +1 of the received word is important. If the error rate of the hard decision value becomes large in the hard decision unit, error necessarily occurs in the mutual correlation operation, and thus the probability of non-detection or erroneous detection of the synchronous word becomes great.
In the case of performing a normalized mutual correlation operation, a correlation operator of FIG. 7 or the mutual correlation operation in step S2 of FIG. 8 may use the following Equation (2) instead of Equation (1)
                              X          n                ←                                            ∑                              i                =                0                                            N                -                1                                      ⁢                                                  ⁢                                          a                i                            ⁢                              y                                                      n                    -                                          (                                              N                        -                        1                                            )                                        +                    i                                    ⁢                                                                                                                                                                ∑                                  i                  =                  0                                                  N                  -                  1                                            ⁢                                                a                  i                  2                                ·                                                                            ∑                                              i                        =                        0                                                                    N                        -                        1                                                              ⁢                                          y                                              n                        -                                                  (                                                      N                            -                            1                                                    )                                                +                        i                                            2                                                                                                                              (        2        )            
In the case of considering the device (that is, hardwared configuration) such as the correlation operator or the synchronous detector, the Equation (2) includes a multiplication operation, square root operation, and division, and thus the amount of operation is increased to that extent, that is, they become the main causes of increasing the hardware scale and the power consumption. In particular, since the square root operation and the division are generally realized using a separate dedicated table, it is frequent to generally increase the hardware scale.
In the situation where the error rate of the hard decision value is great, it is necessary to normalize the mutual correlation operation. In a system where the frequency characteristic (specifically phase characteristic) is greatly changed, the error rate of the hard decision value is increased, and thus a normalization mutual correlation operation is necessary. In many wireless communication systems including wireless LANs, the normalization process is adopted in the mutual correlation operation of the synchronous words (for example, refer to Japanese Unexamined Patent Application Publication Nos. 2008-158855, 2008-72214 and 58-176778).
Here, in the NFC communication, the necessity of normalization of the mutual correlation operation during the detection of synchronous words will be considered.
In the NFC communication, for example, a carrier frequency of 13.56 MHz is used, transmission/reception antennas composed of coils are operated as a pair of transformers, and the communication is performed by the magnetic coupling of the coils. The communication distance is about in the range of 0 mm (close coupled) to around a dozen of cm. As a feature of the transformers, it is given that the respective coils are resonant with the carrier frequency at high Q. By amplifying the signal by resonance in the neighborhood of the carrier frequency, the signal can be transmitted farther away. However, in the case of communicating with two resonated coils, the channel characteristic is greatly changed according to the communication distance.
FIGS. 9A and 9B show frequency characteristics (amplitude characteristic and phase characteristic) of the transmitted signal during data transmission and reception in a state where a distance between antennas composed of coils is in the range of 0.5 mm to 100 mm. For example, in the case where the distance between antennas is 50 mm or 100 mm, one hill having a peak in the neighborhood of the carrier frequency of 13.56 MHz appears. On the other hand, in the case where the distance between the antennas is 20 mm, 6 mm, or 0.5 mm, the resonant hill is separated into two. This is considered to be caused by mutual interference between two close resonant coils. As a result, the carrier frequency of 13.56 MHz becomes a valley between two hills.
In the related art, the change of the channel characteristics according to the communication distance does not specifically become a problem. This is on the basis that the transmission rate being used is not so fast. For example, in the FeliCa and NFC standards, the Manchester code having the transmission rate of 212 kbps is adopted, and the frequency of the maximum repetition wave is 212 kHz (i.e. the transmission band is ±212 kHz). Referring to FIG. 9A, if the communication distance is far away, the receiving antenna level deteriorates by about ½ with respect to the carrier frequency of ±212 kHz. However, in most communication distances, a nearly even frequency characteristic is in the transmission band. Due to this, it is considered that the received signal is not greatly distorted by the frequency characteristic of the channel, and the error rate is suppressed to be low even though the hard decision of {−1, +1} is performed with respect to the received signal.
However, if the transmission rate is heightened even in the NFC communication (as described above), the baseband signal spectrum is widened as much as the heightened rate, and thus the frequency as wide as the heightened rate is necessary during detecting of the received signal. Accordingly, the influence of the frequency characteristic of the channel is increased, the result being to heighten the error rate of the hard decision value of the received words, and this causes the increase of the probability of non-detection or erroneous detection of the synchronous words.
In summary, according to an existing relatively low-speed NFC communication in the related art, since the frequency characteristic in the transmission band is generally even, the error rate of the hard decision value of the received word is decreased, and thus it is not necessary to perform normalization of the correlation operation. In contrast, in the case of the speeding up communication, since the frequency characteristic in the transmission band is not even, the error rate of the hard decision value of the received word is increased, and thus the probability of non-detection or erroneous detection of the synchronous word is also increased.
In a system where the frequency characteristic (particularly, the phase characteristic) is greatly changed, even though the normalization of the mutual correlation has been performed as shown in Equation (2), it is difficult to determine an appropriate threshold value R in all communicable range. That is because it is difficult to estimate the influence that phase shift exerts upon the correlation value in all communication distances.